"It's important for the FDA, the e-cigarette companies and vapers themselves to know that these heating coils, as currently made, seem to be leaking toxic metals -- which then get into the aerosols that vapers inhale," Dr. Ana María Rule, an assistant scientist in the Bloomberg School's Department of Environmental Health and Engineering said in a release.

Vaping allows users to ingest nicotine and enjoy the look and feel of tobacco-smoking, but without the extreme health risks of cigarettes. In E-cigarettes, an electric current passes through a metal coil to heat "e-liquids," creating an aerosol vapor.

Rule and her colleagues, including lead author Pablo Olmedo, recruited 56 daily e-cigarette users from vaping conventions and e-cigarette shops around Baltimore during the fall of 2015.

They tested for the presence of 15 metals in the e-liquids in the vapers' refilling dispensers, the e-liquids in coils' tanks and in the generated aerosols. Four metals were excluded because of low levels: arsenic, titanium, uranium and tungsten. However, among 10 users significant levels of arsenic were found in refill e-liquid, tanks and aerosol samples.

Although minimal levels of metals were found in the e-liquids within refilling dispensers, the metal contamination transferred to aerosols produced by heating the e-liquids.

The median lead concentration in the aerosols was more than 25 times greater than the median in the refill dispensers.

In the aerosol samples, almost 50 percent of them had lead concentrations higher than health-based limits defined by the Environmental Protection Agency. Nickel, chromium and manganese approached or exceeded safe limits, the researchers report.

The source of the lead isn't known, although e-cigarette heating coils typically contain nickel and chromium, among other elements.